a) Field of the Invention
The invention is directed to a process for the transmission of a light bundle which is provided for illuminating picture points of a video picture and has a divergence angle and beam diameter determined by the way in which the light bundle is generated, the light bundle being coupled into a light guide and coupled out with an optical system for bundling a transmitted light bundle exiting the light guide. The invention is further directed to a video system for carrying out the process with a light source for emitting a light bundle for illuminating picture points of a video picture, with a light guide for light transmission into which is coupled a transmitted light bundle with a divergence determined by the light source and with a given beam diameter, and with an optical system for bundling the transmitted light bundle.
b) Description of the Related Art
Video systems in which picture points are illuminated are known, for example, from DE 43 24 848 C1. In these video systems, at least one light bundle is deflected in a scanning pattern over a screen in accordance with a video standard as is known in television from the generation of pictures by means of an electron beam with the deflecting coils provided for scanning. The light bundle is intensity-modulated in a manner analogous to the above-mentioned electron beam in order to generate a gray value or color corresponding to the picture content for the individual picture points shown on the screen. Laser beams are customarily used as light bundles because they are parallel and also permit a high resolution even where there is a great distance between the screen and the scanning device.
It is proposed in DE 43 24 848 C1 to spatially separate the laser beam scanning arrangement behind the laser modulating device in the light propagation direction. This is advantageous for maintenance and/or for a particularly favorable dividing up of space in an especially compact laser video projection system. In addition, the use of a light waveguide is suggested for transmitting laser beams between the laser modulating device and the scanning device. Light-conducting fibers in which the light is guided in a sheathed core are especially known as light waveguides. Unwanted emergence of light from the core is prevented by means of total reflection between the core and cladding.
However, the high degree of parallelism of the coupled-in laser beam is eliminated in light-conducting fibers because of variable reflection between the core and cladding, so that the light emerging from the light-conducting fiber has a substantially higher divergence than the coupled-in laser beam. In order to reduce this divergence, it is further proposed in DE 43 24 848 C1 to provide an optical system behind the light-conducting fiber in the light propagation direction, which optical system parallelizes the light bundle exiting from the light-conducting fiber.
In spite of this, it has been shown that, in practice, the light bundle which is parallelized in this way has inferior beam characteristics compared with the light that is coupled into the light-conducting fiber. Complete parallelism cannot be achieved.
However, for a high-resolution video picture, especially for the impending HDTV standards, a small divergence is required in order that the laser light spot which substantially determines the picture point size is so small on the screen that it corresponds to the required line resolution. As was shown in the foregoing discussion, the divergence is greatly increased when a light-conducting fiber is used. Although it could also be attempted to focus the coupled-out laser beam on the screen rather than parallelize it, it would be necessary to surrender the advantage resulting from a virtually parallel light beam, namely that a sharp picture is always obtained without changing the focussing conditions when the distance between the screen and scanning device changes. This advantageous characteristic of a virtually parallel light bundle for illuminating picture points allows a sharp picture to be shown on objects of any shape, for example, which is impossible with other projectors.
Transmission within light-conducting fibers also has a decisive influence especially on the mode structure of the laser beam because the single-mode structure of the laser is resolved into a plurality of modes when multimode light-conducting fibers are used. According to the teaching of DE 43 24 848 C1, it would be possible to parallelize an individual mode of the occurring modes; however, the other modes would then be expanded, so that for the entirety of the light bundle emanating from the light-conducting fibers there would be no resolution suitable for high-resolution video pictures.
Further, according to DE 43 24 848 C1, a collecting lens whose focal point coincides with the center of the light exit face is arranged at the light exit face of the light waveguide. Accordingly, it is achieved in theory that the point of the light exit face containing the optical axis of the collecting lens is imaged in infinity. However, since the light exit face of the light waveguide has a finite extension, parallelism cannot be achieved for the propagation condition of the laser light which exits over the entire cross-sectional surface of the light waveguide, for example, also in the vicinity of the cladding of a light-conducting fiber. Even the imaging of laser beams emerging parallel to the optical axis by means of the lens can also sharply decrease the achievable resolution due to the light bundle which is expanded by this lens and which strikes the screen.